The San Andreas Fault system is seen in this photo. (Credit: Carol M. Highsmith/Buyenlarge/Getty Images)

Southern California gets a lot more action than we thought.

In a new study, researchers from the California Institute of Technology and University of California, San Diego have uncovered nearly two million previously unidentified tiny earthquakes that happened between 2008 and 2017. That translates to roughly 495 earthquakes a day in Southern California, or an earthquake about every three minutes.

An average person walking along the street wouldn’t notice these tiny temblors, which range in magnitude between -2.0 and 1.7.

Using larger earthquakes to find smaller ones

Even researchers who study earthquakes had a difficult time detecting these “hidden” earthquakes.

Background noise, such as shaking from automobile traffic or building construction, appears in seismic data and can mask signs of the smaller earthquakes.

To detect these elusive quakes, the scientists used a technique called template matching. From seismic data of already known earthquakes, they identified patterns as to what an earthquake’s signal should look like. Using this information, the researchers then scanned records from seismometers to find the tiny earthquakes.

The template matching method has been used in seismology since about 2006, but it was mostly limited to analyzing small data sets for a couple weeks at time.

“The computational burden of using this method is heavy,” said Zach Ross, lead researcher on the study. “It requires big computers, which pretty much limited its usage to small chunks of data. When we started on this project, we wanted to apply this on a scale … significantly larger than anything that’s ever been done before.”

Scientists now have a lot of data to learn from

The expanded earthquake data will help scientists understand how swarms of earthquakes evolve and previously unknown temblors that precede major earthquakes.

“The reason we’re interested in smaller earthquakes is because we don’t have enough large quakes on record to look at the long-term evolution of earthquakes and faults,” Ross said. “The (smaller earthquakes) start to fill in all the gaps between the larger ones.”

Marine Denolle, a seismologist at Harvard University who was not involved in the study, said she is excited about the future studies that this newfound data will generate.

“This is the biggest earthquake catalog ever,” Denolle told CNN. “This will help us find where earthquakes come from, and they can highlight new fault systems that we could not see before and reveal new tectonics that we were not aware of before.”